On Sun, Sep 25, 2011 at 9:40 AM, David Winsemius <dwinsem...@comcast.net> wrote:
> (I do not know why Sarah Goslee is telling you that you cannot access > matrices with column names. You clearly can do so. Maybe I misread her > statement.) I suppose I should have said that you can't access matrices via column name *like that*. A data frame is a list, so it can be accessed in the way in which the original querent was trying. And the latter is by no means the "wrong way" to do so, merely a way that cannot be used with matrices. David points out the correct way to access columns of a matrix by name. But do note that the default results of x["a"] and x[,"a"] differ, even if x is a data frame. > x <- cbind(a=1:3, b=11:13) > class(x) [1] "matrix" > x["a"] [1] NA > x[,"a"] [1] 1 2 3 > > x <- data.frame(x) > class(x) [1] "data.frame" > x["a"] a 1 1 2 2 3 3 > x[,"a"] [1] 1 2 3 > x[["a"]] [1] 1 2 3 > > class(x["a"]) [1] "data.frame" > class(x[,"a"]) [1] "integer" > class(x[["a"]]) [1] "integer" > > class(x[,"a", drop=FALSE]) [1] "data.frame" > There are several entirely valid ways to access named columns, and they're all slightly different. Which is guaranteed to confuse the new R user, really. Sarah -- Sarah Goslee http://www.functionaldiversity.org ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
